fix(gui): address ds-review-bot findings on image attachments

- dsh web gains --provider/--model: a non-deepseek provider mounts the
  matching pi-ai catalog route (ambient credentials), making image input
  reachable from the shipped Web assembly; requires an explicit --model
- attachment-local syncs the publication directories after the hard-link
  publish so a reported durable reference survives a crash (POSIX; Windows
  relies on filesystem metadata journaling)
- the attachment seam gains storage-free validateImage; the host validates a
  complete multi-image prompt before persisting any member, so one malformed
  image cannot strand valid members as unreferenced objects
- startSession sends before navigating: a rejected first send keeps the empty
  state, its error strip, and the complete draft mounted
- the webserver rejects an undeclared-length body the moment it crosses the
  configured limit instead of draining a potentially endless stream to EOF

packages/cordis/tool-cordis/src/api-catalog.ts

@@ -156,6 +156,10 @@ export const SERVICE_API: readonly ServiceApiEntry[] = [
     key: 'attachments',
     summary: 'Immutable binary attachment service.',
     methods: [
+      {
+        signature: 'abstract validateImage(input: SaveImageAttachment): void',
+        jsDoc: '/**\n * Validate one image against the deployment policy without persisting anything.\n * Callers persisting a multi-image batch validate every member first so a\n * malformed member cannot leave earlier members as unreferenced objects.\n * @param input - encoded bytes, declared media type, and optional display name.\n */',
+      },
       {
         signature: 'abstract saveImage(input: SaveImageAttachment): Promise<ImageAttachmentRef>',
         jsDoc: '/**\n * Validate and durably commit one image before its owning session event is appended.\n * @param input - encoded bytes, declared media type, and optional display name.\n * @returns a durable content-addressed reference.\n */',